This invention relates to inverters, and more particularly, to a pulse stretching circuit for use in conjunction with an auxiliary commutation circuit to aid in the commutation of the main thyristors of an inverter during transient or overcurrent conditions.
2. Description of the Prior Art
Inverters are known generally and are devices which transform DC (direct current) electrical energy, such as from a fuel cell or the like, into AC (alternating current) electrical energy suitable for use by utility companies or other consumers of electrical energy. Most inverters include at least one pair of main switching elements, and by alternatively actuating each switching element, electrical energy from the DC source flows through a first load in one direction and then in the reverse direction forming a fundamental AC waveform.
Numerous different types of switching devices can be employed in an inverter as a switching element to reverse the current through the load. Semiconductor switches, such as thyristors, are frequently used in present day inverters and this type of device is substantially unidirectional so that the high energy current pulses pass in only one direction through the semiconductor switch from the input terminal to the output terminal when the switch is turned on by a control signal. Some semiconductor switches, as is known, will not immediately change from a conducting state to a nonconducting state upon the removal of the control signal from the control terminal, but require that the magnitude of the instantaneous current passing therethrough be reduced to near zero allowing the semiconductor switch to transition to its off state.
The process by which the current is reduced to zero is known as "commutation" and numerous circuit configurations have been proposed for this function. Many commutation circuits operate by presenting a commutation pulse to the load from a storage device, such as a capacitor or resonant circuit, for a period of greater than the turn off time of the semiconductor switch. Since during this period the load current is supplied by the storage device of the commutation circuit, the magnitude of the current to the semiconductor switch drops to zero for sufficient period to allow transition to the nonconducting state.
It is well known in the art that the amount of energy stored in the commutation capacitors is a function of the value or capacitance of the capacitors as well as the voltage impressed on the capacitor; however, the amount of stored energy required to commutate the main semiconductor switches is proportional to the magnitude of the current therethrough, i.e. the greater the magnitude of the load current, the more stored energy required to commutate the semiconductor switches. Accordingly, the value of the commutation capacitor or capacitors is often selected by ascertaining the highest value of load current which must be commutated, and then sizing the commutation capacitors such that the necessary commutation pulse can be provided.
Prior art inverters are also known which include a commutation circuit having an auxiliary portion which is only operative during overcurrent conditions. The amount of electrical energy stored and discharged during each commutation cycle is reduced in that only the amount of energy required for a normal commutation must be stored. However, during a transient or other overcurrent condition which results in a high instantaneous value of load current during the commutation period for the main semiconductor switches, the additional energy stored in appropriate portions of the auxiliary commutation circuit is gated on to supplement the normal commutation pulse.
A particular problem with an auxiliary commutation circuit which switches additional capacitance into a normal commutation cycle is that the variation in resonant frequency which occurs as a result of the change in capacitance must be considered in the sequencing of the switches. Because the natural period of the commutation current pulses has been varied, the zero crossing point of the commutation current pulse no longer coincides with the original thyristor switch point and the makeup pulse begins prematurely which causes a makeup pulse of excessive amplitude.
Of particular interest in U.S. application Ser. No. 930,469 of J. P. Vivirito filed on Aug. 2, 1978 entitled AUXILIARY COMMUTATION CIRCUIT FOR AN INVERTER assigned to the same assignee as the present invention, which discloses an auxiliary commutation circuit of the impulse commutated bridge inverter type in which additional commutation energy is stored on a pair of oppositely charged capacitors. Switching elements in series with the capacitors are operable in response to a sensed overcurrent condition to provide additional stored energy during commutation.
Also of particular interest is U.S. Pat. No. 3,249,844 issued May 3, 1966 to J. Jensen for SEMICONDUCTOR APPARATUS in which the load current of an inverter is sensed to control switching elements for introducing additional commutating capacitors as the load current demand increases. It is important to note that these auxiliary capacitors (items 30-32 in the drawing) are coupled into the inverter circuit in parallel with the continuously operating capacitor (item 27 of the drawing) when the auxiliary capacitors are in an uncharged state. This is significant because without stored energy in the auxiliary capacitor, the increased amount of energy required to extinguish a transient current through a thyristor is not available and the thyristor cannot be commutated to its off state. In fact, the introduction of an uncharged auxiliary capacitor into the commutation circuit at a time when a transient condition occurs can have a significant adverse effect in that the charge required by the auxiliary capacitor diminishes the commutation energy available for extinguishing the main thyristor.
Of interest is U.S. application Ser. No. 936,277 by J. Messer et al. filed on Aug. 23, 1978 entitled TWO-STAGE COMMUTATION CIRCUIT FOR AN INVERTER which discloses a commutation circuit having at least two portions with different energy storage capability, each of which is suited to a particular level of input voltage. One portion of the commutation circuit is suitably sized to commutate the magnitude of the load current at lighter loads while the second portion of the commutation is sized to commutate the thyristor current during higher load current levels. A control circuit is described for sensing the levels of the DC input voltage and current detector to allow the transition between the two portions of the commutation circuit at a time interval in which the load current is essentially zero.